1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more specifically, to a method of manufacturing a silicon carbide (SiC) semiconductor device having a high resistance silicon carbide monocrystal layer which functions as an electric insulating layer or functions as an electric insulating layer and a channel layer. Particularly, it relates to a method of manufacturing an insulated gate field effect transistor and a Schottky field effect transistor for high temperature operation, whose device characteristics are not deteriorated until the temperature reached a high temperature range about 500.degree. C.
2. Description of the Prior Art
Semiconductor devices (e.g., a diode, a transistor, an integrated circuit, a large scale integrated circuit, a light emitting diode, a semiconductor laser and a charge coupled device) using compound semiconductors such as silicon (Si) and others like gallium arsenide (GaAs), gallium phosphide (GaP), etc., have been developed generally in every field of electronics.
Silicon carbide (SiC) is a semiconductor material having a large forbidden band width (2.2 to 3.3 eV) and has an outstanding characteristic that it is thermally, chemically and mechanically extremely stable and withstands radiation damage. It is difficult to use semiconductor device employing a conventional semiconductor material such as silicon, under the severe condition like high temperature, high output drive, radiation irradiation, etc. Accordingly, a semiconductor device employing silicon carbide is expected to be applied in variety of fields as a semiconductor device which can be used under such severe conditions.
However, crystal deposition technique by which silicon carbide monocrystal of a large area and of high quality is produced stably on the basis of industrial scale allowing for productivity has not been established. Therefore, silicon carbide has not been practically used although it is a semiconductor material having various advantages and possibility as mentioned above.
Conventionally, on the basis of investigation in laboratories, a silicon carbide monocrystal of a size at which an experimental prototype of a semiconductor device can be manufactured is obtained by depositing silicon carbide monocrystal by sublimate recrystalization method (Rayleigh method) and depositing silicon carbide monocrystal layer on a substrate of the silicon carbide monocrystal thus obtained, by chemical vapor deposition method (CVD method) and liquid phase epitaxial growing method (LPE method). However, with these methods, obtained monocrystal is small in area and it is difficult to control its size and shape with high accuracy. Moreover, it is not easy to control crystal polymorphism and impurity concentration of silicon carbide. Thus, a technique of manufacturing a semiconductor device employing the silicon carbide monocrystal is anything but a practical manufacturing method on the basis of industrial scale.
To solve these problems, the inventors of the present invention proposed a method of depositing silicon carbide monocrystal of a large area and of good quality on a silicon monocrystal substrate which is cheap and commercially available (Unexamined Japanese Patent Publication No. 203799, 1084). This method employs a technique of forming a silicon carbide thin film on a silicon monocrystal substrate by low temperature CVD method and depositing silicon carbide monocrystal by CVD at higher temperature. With this method, implanting impurity in depositing silicon carbide by means of CVD makes it possible to control impurity concentration and conductivity type of the obtained silicon carbide monocrystal.
Utilizing the silicon carbide monocrystal layer formed on the silicon monocrystal substrate by this method, methods of manufacturing various semiconductor devices (e.g., a diode and a transistor) has been developed. When a plurality of semiconductor devices of various types should be made in the silicon carbide monocrystal on the silicon substrate, there are many cases where electrical isolation among devices is required, and it is desirable that the devices are made on the silicon monocrystal substrate having high electric resistance. However, with the above-mentioned chemical vapor deposition method, the silicon carbide monocrystal layer having high electric resistance can not be obtained.
Accordingly, the inventors proposed a method of forming a silicon carbide film of high electric resistance by implanting boron to a silicon carbide monocrystal in a process of forming this silicon carbide monocrystal on a silicon substrate by chemical vapor deposition (Unexamined Japanese Patent Publication No. 264399, 1985). In this method, the temperature of the silicon carbide monocrystal substrate is kept at about 1,350.degree. C., and mono-silane (SiH.sub.4) and propane (C.sub.3 H.sub.8) as ingredient gases are supplied about 0.4 cc per minute. At the same time, diborane (B.sub.2 H.sub.6) as a gas for impurity is supplied 0.02 cc per minute with hydrogen carrier gas (3 liters per minute) from a branch pipe. Thus, deposition is carried out for one hour. As a result, a high resistance silicon carbide monocrystal film having a resistivity of 600 .OMEGA..multidot.cm is obtained on the entire surface of the silicon monocrystal substrate with a thickness of about 2 .mu.m.
In other words, the silicon carbide monocrystal film is deposited on the silicon monocrystal substrate by supplying the ingredient gas consisting of mono-silane and propane, and boron is added to the silicon carbide monocrystal film as impurity by simultaneously supplying diborane.
In general, in these silicon carbide semiconductor devices (e.g., a field effect transistor), an n-type (or p-type) silicon carbide monocrystal layer to be a channel layer is formed on a p-type (or n-type) silicon carbide monocrystal layer by chemical vapor deposition method, using boron, Al or the like as impurity so that the pn junction of these layer electrically insulate the channel layer from the semiconductor substrate. However, in the semiconductor device having thus structured, when reverse bias voltage is applied to the pn junction, leak current is generated between the semiconductor substrate and the channel layer due to crystal defects existing in the silicon carbide monocrystal layer. Accordingly, it is difficult to completely electrically insulate the channel layer from the semiconductor substrate, and thus good transistor characteristic can not be obtained. Even if the thickness of the silicon carbide monocrystal layer deposited by chemical vapor deposition is made larger to avoid an influence of the crystal defects, the crystal defects still exercise an influence, and the leak current is generated.
Conventionally, a high resistance silicon carbide monocrystal layer has been used as an electric insulating layer between a semiconductor substrate and a channel layer instead of the above-mentioned pn junction between silicon carbide monocrystal layers, so as to electrically insulate the semiconductor substrate and the channel layer.
However, when especially an insulated gate field effect transistor (MOSFET) is formed using silicon carbide monocrystal, a channel layer is formed on an electric insulating layer, and thereafter drain and source regions must be provided in the channel layer. On the channel layer, a gate insulating film must be further formed. Thus, in order to simplify the manufacturing process, it is considered that a single high resistance silicon carbide monocrystal layer serves as both the electric insulating layer and the channel layer, but in this case, the problem is a method of manufacturing a silicon carbide monocrystal layer having a sufficient resistance to function as both the electric insulating layer and the channel layer.
In the case where a high resistance silicon carbide monocrystal layer is used as an electric insulating layer having a channel layer of a SiC monocrystal layer on an upper part in a Schottky field effect transistor (MESFET), the problem is a method of forming the silicon carbide monocrystal layer.
Conventionally, as such forming methods, for example, boron may be added as impurity in depositing a SiC monocrystal layer by the above-mentioned chemical vapor deposition, or boron, or impurity, may be thermally diffused into a SiC monocrystal layer deposited in advance.
However, boron added into the SiC monocrystal layer in the chemical vapor deposition is not completely ionized at the room temperature, but the carrier concentration in the silicon carbide monocrystal layer increases as the temperature rises. That is, as shown in FIG. 5, the resistivity of the silicon carbide monocrystal layer lowers as the temperature rises. (See A. Suzuki et al., Appl. Phys. Lett., 49, 450 (1986) and M. Yamanaka et al., J. Appl. Phys., 61, 599 (1987), for example.) Therefore, these methods are inappropriate to manufacture a high resistance SiC monocrystal layer satisfactory to a SiC semiconductor device for high temperature operation at a high temperature range of 200.degree. C. to 500.degree. C., even if it were made as a high resistance SiC monocrystal layer which functions as an electric insulating layer or functions as an electric insulating layer and a channel layer (the electric insulating layer serves as the channel layer). Additionally, since the SiC monocrystal is deposited by chemical vapor deposition with impurity being added, many concave and convex portions are caused on the surface of the deposited layer of the SiC monocrystal. Especially, in a MOSFET, the surface of a channel layer of silicon carbide monocrystal decreases in flatness, and adverse effect is exerted on a characteristic of a gate insulating film formed on the channel layer.
On the other hand, when a high resistance layer is formed by thermally diffusing boron as impurity into a silicon carbide monocrystal layer deposited in advance, a constant of impurity diffusion in silicon carbide is small, and high diffusing temperature of 1600.degree. C. or more is required. Accordngly, the the method thermally diffusing impurity is inappropriate as a process technique of a silicon carbide semiconductor device, because it is difficult to control impurity concentration, and the used semiconductor substrate and silicon carbide monocrystal layer may be deteriorated. Moreover, with both the methods, crystal defects undeniably exert an influence.
The present invention solves the above problems in the conventional embodiment, and it is an object of the present invention to provide a silicon carbide semiconductor device having a high resistance silicon carbide monocrystal layer which is easily controlled in its impurity concentration and functions as an electric insulating layer or as an electric insulating layer and a channel layer even in a high temperature range around 500.degree. C., for example.
Thus, the present invention provides a method of manufacturing a semiconductor device, comprising the steps of (i) forming a SiC monocrystal layer over the entire surface of a semiconductor substrate; (ii) forming a boron ion implanted layer, which is substantially a thin film, by implanting a specified amount of boron ions in the surface region of the SiC monocrystal layer; and (iii) forming a high resistance SiC monocrystal layer of a thin film by subjecting the boron ion implanted layer to heat treatment; whereby the high resistance SiC monocrystal layer can be function at least as an electric insulating layer.